Polymers obtained by a ring-opening polymerization of cycloolefins that contain the norbornene moiety are well known. For example, U.S. Pat. Nos. 4,136,249; 4,178,424; 4,136,247 and 4,136,248, assigned to the same assignee as the present invention, describe such polymers and each is incorporated herein by reference for the description of polymers therein.
Ring-opening polymerization of cycloolefins yields unsaturated linear polymers which are a particular interest in that they are known to be reactive (sulphurvulcanizable) and with difunctional monomers, branching and cross-linking can occur. They are known to exhibit attractive property profiles for many polymer applications, such as, for example, as automotive parts, particularly decorative trim.
The ring-opening polymerization reaction has been found to be sufficiently rapid to be amenable to bulk polymerization techniques such as, for example, reaction injection molding and reagent transfer molding. Minchak in U.S. Pat. No. 4,426,502, discloses a bulk polymerization process for "norbornene-type monomers" which include norbornene, dicyclopentadiene, tricyclopentadiene (cyclopentadiene trimer), tetracyclododecene and other cycloolefin monomers containing a norbornene functional group. Bulk polymerization processes provide the advantage of synthesizing polymers in a mold in the final shape desired so that no further melt processing is required. Therefore, high molecular weight polymers with high glass transition temperatures can be synthesized without concern for the need of extreme process conditions to process these polymers into finished articles.
Dicyclopentadiene is a common cycloolefin monomer used to prepare ring-opened polymerized polymers in that it is readily available as a by-product in ethylene production. Dicyclopentadiene is a suitable monomer for bulk polymerization techniques, as disclosed by Minchak in U.S. Pat. No. 4,426,502. For such bulk polymerizations, liquid reagents are preferred in that they are handled easily in solids, provided they are not too viscous.
Rapid mixing is a desired feature for bulk polymerizations of cycloolefin monomers in that the reaction generally proceeds in the presence of a two component catalyst/cocatalyst system. At least one component remains isolated from the monomers until polymerization is desired. Once a complete catalyst/cocatalyst system is dissolved within such monomers, ring-opening polymerization occurs. The reaction rate may be enhanced by increasing the temperature of the reacted mixture. A common procedure is to combine the catalyst components in two separate volumes of cycloolefin monomer and mix these two volumes prior to injection into a mold. Such a technique typically commands the use of monomer in liquid form.
Problems arise with the use of dicyclopentadiene in that it is a solid at ambient temperature when sufficiently pure for use in bulk polymerization reactions. The melting point for high purity dicyclopentadiene is generally above about 31.degree. to 32.degree. C. Although dicyclopentadiene can be rendered liquid with slight heating, this high melting temperature presents a significant disadvantage in commercial processes. For example, in a continuous system, feedlines for monomer must be heated to prevent solidification of the dicyclopentadiene. In addition, when shipping the monomer, considerable trouble and expense is realized in melting the monomers when they arrive at their destination for use in polymerization processes.
The technique of melting point/freezing point depression of a pure composition by the addition of one or more soluble components is well known. However, high purity dicyclopentadiene has been found to be necessary for bulk polymerization processes to obtain useful products. Impurities will often provide liquid dicyclopentadiene mixtures but will also retard polymerization. Adding an inert solvent or diluent will adversely affect the products obtained in that the unreacted component will reduce impact properties and/or leach from the finished article, rendering it useless. It is desirable to identify species which will lower the freezing point of dicyclopentadiene without inhibiting the properties obtained upon ring-opening polymerization in bulk. While additives such as impact modifiers have been commonly used in feedstocks of dicyclopentadiene for ring-opening bulk polymerization, these additives generally do not depress the melting point/freezing point of dicyclopentadiene. In addition, while blends of dicyclopentadiene with other cycloolefin monomers have been described as suitable for use in bulk polymerization reactions by Minchak in U.S. Pat. Nos. 4,380,617 and 4,426,502, methods which efficiently reduce the melting point/freezing point of dicyclopentadiene without adversely affecting the properties of polymers obtained on bulk polymerization, have not been described. The methods provided by this invention permit high levels of dicyclopentadiene to be used in forming liquid dicyclopentadiene mixtures.